In some fields, there is a demand for a moving-picture distribution system in which a server is capable of distributing the same source such as a moving picture at the same time to a plurality of terminals having capabilities different from each other. The fields include the field of streaming distribution of information such as a video on demand or a live picture and the field of real-time communication applied to a video conference, a television telephone or the like. Examples of the terminals are a cellular phone having a display device with a low resolution as well as a CPU (Central Processing Unit) with a small processing capability and a personal computer having a display device with a high resolution as well as a CPU with a large processing capability.
As such a moving-picture distribution system, a moving-picture distribution system, which makes use of a layered encoding technology of carrying out a layered encoding process on data of moving picture typically in frame units, currently becomes a subject of research and development. Compression/decompression systems capable of utilizing this layered encoding technology include the already known MPEG (Moving Picture Experts Group)-4 system and Motion JPEG (Joint Photographic Experts Group)-2000 system for handling not only still pictures but also moving pictures.
In the MPEG4 system, a layered encoding technology known as a Fine Granularity Scalability technology is included in its standard and planned to be profiled. This layered encoding technology is said to be a technology that is capable of distributing a moving picture in a scalable manner at different bit rates ranging from a higher one to a lower one.
On the other hand, the Motion JPEG2000 system adopts a wavelet transform technology. In this system, frame data is processed by adopting a packet generation technology and the layered encoding technology utilizing characteristics of the wavelet transform technology. To put it in detail, techniques are adopted for encoding the data to provide predetermined qualities and arranging pieces of encoded data in a predetermined order before putting the data in layered packets. The qualities include a spatial resolution and an SNR (Signal to Noise Ratio) picture quality, which is referred to hereafter simply as a picture quality.
In addition, in accordance with Part 3 of the Motion JPEG2000, layered data is kept in a file format.
Thus, in the conventional moving-picture distribution system, the server needs to prepare a sender data as a plurality of pieces of moving-picture data for distribution with different formats for terminals with different capabilities or a plurality of different pieces of data for different transmission rates. In a moving-picture distribution system adopting a layered encoding technology like the one described above, however, the server is capable of distributing data at the same time to terminals with different capabilities by merely preparing a piece of file data put in a layered structure.
When adopting the Motion JPEG2000 method in the moving-picture distribution system, however, the Motion JPEG2000 is used as a base and one server distributes data of a moving picture to a plurality of terminals. In this case, the moving-picture distribution system raises a problem that the aforementioned piece of file data put in a layered structure must be processed in ways different from each other in dependence on qualities specified by the terminals.
Assume for example that a first terminal requests a server to transmit data of a moving picture, specifying a spatial resolution progressive. On the other hand, a second terminal requests the server to transmit the data of the moving picture, specifying a picture-quality progressive.
In this case, the server typically carries out a layered encoding process on the data of the moving picture in frame units and packetizes the encoded data to generate one piece of file data. A piece of file data includes a plurality of packets. Thus, the moving-picture distribution system raises a problem that, as post-processing, the server needs to concurrently carry out the following different processes for the different progressives.
To be more specific, the server must carry out complicated processing as follows. First of all, the server examines the packets composing the file data to discriminate packets for a specific one of the progressives against packets for the other progressive. Then, the packets for the specific progressive are rearranged in the order of this specific progressive and the packets for the other progressive are rearranged in the order of the other progressive to generate a group of packets for each of the progressives specified by the terminals. Finally, the server selects a group of packets for each of the terminals and transmits each selected group of packets to the appropriate terminal. As a result, the moving-picture distribution system raises a problem that it is difficult to carry out this processing in a real-time manner.
It is to be noted that, data is encoded in accordance with a predetermined encoding method and pieces of encoded data arranged in a predetermined order are decoded and displayed in a display in which, initially, a picture having a low quality is displayed and, with the lapse of time, the displayed picture is improved for higher quality. In this specification, such a display is-referred-to as a progressive display, an encoding method for producing a progressive display is known as a progressive encoding method, and an order in which codes are arranged to implement a progressive display is called a progressive order.
That is to say, the encoding method based on the Motion JPEG2000 system, that is, the encoding method utilizing the wavelet transform, is an implementation of the progressive encoding method. In other words, a Motion-JPEG2000 encoder encodes data of a moving picture in frame units in such a way that a picture obtained as a result of a decoding process carried out by a decoder serving as a counterpart of the encoder is displayed in a progressive display.
In addition, the quality of a picture appearing in a progressive display is referred to hereafter as a progressive. To put it concretely, the progressive (quality) can be expressed in terms of a spatial resolution, a picture quality and color components to mention a few.
Incidentally, Sony Corporation serving as an applicant for this patent has developed a moving-picture distribution system adopting the Motion JPEG method described above to assign pictures of different qualities in frame units to a plurality of terminals and distribute the frames to the terminals as disclosed in Japanese Patent Laid-open No. 2000-341668.
To put it concretely, an encoder employed in a server of the moving-picture distribution system is capable of encoding frames on the basis of a frame interval set in advance and a quality varying from frame to frame. Thus, the server is capable of distributing a moving picture having a quality corresponding to one of a plurality of terminals receiving the moving picture. The quality varies from frame to frame in dependence on the display capability of the receiver terminal, a frame rate specified by the terminal or a usable band of the network connecting the server to the terminal.
In this moving-picture distribution system, however, only one type of quality is assigned to every frame. Thus, given a server capable of encoding moving-picture data at a rate of 30 frames per second, for example, the server apportions 15 frames of the 30 frames to a terminal specifying a moving picture with a resolution of 15 frames per second and 5 frames of the remaining 15 frames to another terminal specifying a moving picture with a resolution of 5 frames per second.
That is to say, the number of terminals that can receive a distributed moving picture is determined by how the server apportions these 30 frames to the terminals. Thus, this moving-picture distribution system has a problem that the number of terminals that can receive a distributed moving picture with different qualities is limited.
In addition, this moving-picture distribution system raises another problem that, by apportioning 15 frames to the terminal specifying a moving picture with a low resolution of 15 frames per second, the moving-picture distribution system cannot apportion the remaining 15 frames to a terminal specifying a moving picture with a high resolution of 30 frames per second due to the fact that its encoding capability is only 30 frames per second. This is because a frame of an assigned quality can be used only at the assigned quality.